2.2(a+b)- gas exchange in plants and animals

• Cuticle function

  • a transparent, waxy layer secreted by the epidermis. the cuticle reduces water loss through the epidermis. since it is transparent it allows light to pass through to the layers below.
• epidermis function

  a single layer of transparent cells forming a surface layer over the leaf. the epidermis prevents mechanical damage and helps to prevent bacteria and fungi penetrating the leaf
• palisade mesophyll function

  • layer made up of one or several layers of cells. the cells are elongated, with their long axes perpendicular to the surface of the leaf, and they are densely arranged. they contain many chloroplasts which are able to move within the cytoplasm to absorb the maximum amount of light. the cells contain large vacuoles which push the cytoplasm, with the chloroplasts, towards the outside of the cells. this is the main site of photosynthesis.
• spongy mesophyll

  • some light passes through to the spongy mesophyll. The cells here are irregularly shaped and have large air spaces between them. the spaces between the cells allow co2 to diffuse rapidly to the cells of the palisade mesophyll and o2 can diffuse away. the cells are moist so gases can dissolve. they contain chloroplasts, so play a role in photosynthesis.
• vascular bundle

  • the leaf contains a network of veins providing support for the leaf. within each vein, xylem transports h2o and minerals to the cells of the leaf and phloem transports the products of photosynthesis ( mainly sucrose ) to other regions of the plant
• stomata and guard cells


  the presence of a waterproof cuticle greatly reduces the ability of a leaf to exchange gases; they have therefore evolved pores or stomata which allows gases to diffuse into and out of their leaves. each stoma is surrounded by a pair of guard cells which control the opening of the stoma by changes in their turgor.
• adaptations of the leaf of photosynthesis 


  gases diffuse through the stomata along a concentration gradient.
• to enable gas exchange to take place efficiently: LEARN!!!

  1. large surface area-allows room for many stomata
  2. leaf blade is thin- shorten diffusion distance of gases entering and leaving
  3. spongy mesophyll has many air spaces- allows diffusion of co2 and o2 between the stomata and the cells
  4. cells are moist-gases can dissolve
  5. stomatal pores permit gas exchange
• adaptations to allow max. absorption of light

  1. large surface area- absorbs much as light possible
  2. thin- allows light to penetrate lower layer of cells
  3. cuticle and epidermis are transparent- allows light to penetrate to the mesophyll.
  4. palisade cells are elongated and densely arranged in a layer or layers- allows many palisade cells to be packed in.
  5. palisade cells are packed with chloroplasts- to absorb as much light as possible.
  6. chloroplasts can move and rotate-for the maximum absorption of light
• structure of angiosperm leaf

  Label the structure

  A) lower epidermis

  B) spongy mesophyll

  C) palisade mesophyll

  D) upper epidermis

  E) cuticle

  F) vascular bundle

  G) phloem

  H) xylem

  I) stoma

  J) guard cell

  
• Stomatal opening

  1. Chloroplasts in the guard cell photosynthesise, producing ATP

  2. ATP provides the energy for active transport to move potassium ions from the surrounding epidermal cells into the guard cells

  3. Starch is turned into the sugar malate

  4. The potassium and malate lowers the water potential in the guard cells- makes it more negative

  5. Water moves into the guard cells from nearby cells by osmosis

  6. The guard cells swell and become turgid, they curve apart because the cell wall of the guard cell on the inner side of the stomatal pore is thicker and less elastic than the thin wall of the guard cell further from the stomatal pore

  7. This opens the stomatal pore
• stomatal opening- night

  1. atp provides energy for active transport to move k+ ions out of the guard cells.
  2. malate is converted back into starch
  3. water potential increases in the guard cells
  4. Water moves out of the guard cells by osmosis.
  5. guard cells become flaccid and straighten. stomatal pore closes.

  
• stomata close:

  1. at night, to reduce water loss when there is less light for photosynthesis
  2. In very bright light, as there is intense heat, which would increase evaporation.
  3. If there is excessive water loss.
• For rapid diffusion of gases, a respiratory surface must

  Have a large enough surface area, relative to the volume of the organism, so that the rate of gas exchange satisfies the organisms need.

  Be thin, so diffusion pathways are short.

  Be permeable so that the respiratory gases diffuse easily

  Have a mechanism to produce a steep diffusion gradient across the respiratory surface, by bringing oxygen or removing carbon dioxide, rapidly.
• Single celled organisms have

  A large surface area to volume ratio

  Cell membrane is thin so diffusion into the cell is rapid.

  It is also thin so diffusion distance inside the cell are short.
• Single celled organisms can

  Absorb enough oxygen across the cell membrane to meet their needs

  Remove carbon dioxide fast enough to prevent building up a high concentration and making the cytoplasm too acidic for enzymes to function.
• Amoeba


  It has a thin membrane so diffusion is rapid. It also has a large surface area to volume ratio therefore absorbs oxygen across the cell membrane to meet their needs for respiration.
• Flatworm


  Being flat they have a much larger surface area. Their large surface area to volume ration has overcome the problem of size increase because no part of the body is far from the surface and so diffusion paths are short.
• Earthworm


  Its cylindrical and so its surface area to volume ratio is smaller than a flatworm. Its skin is the respiratory surface, which it keeps moist. It has low oxygen requirements because it is slow moving and had low metabolic rate.
• Major problems for terrestrial organisms are

  Water evaporates from body surfaces, which could result in dehydration.

  Gas exchange surfaces must be thin and permeable with a large surface area. But water molecules are very small and pass through gas exchange surfaces, so gas exchange surfaces are moist.
• Amphibians


  Their skin is moist and permeable, with a well developed capillary network just below the surface. Gas exchange takes place through the skin and, when the animal is active, in the lung also.
• Reptiles


  Their lungs have a more complex internal structure than those of amphibians, increasing the surface area for gas exchange.
• Birds


  Their lungs process a large volume of oxygen because flight requires a lot of energy. Birds do not have a diaphragm, but their ribs and flight muscles ventilate their lungs more efficiently.
• Parallel flow


  Blood and water flow in the same direction at the gill Lamellae, maintaining the concentration gradient for oxygen to diffuse into the blood only to the point where its concentration in the blood and water is equal.
• The ventilation system in cartilaginous fish is less efficient than that of bony fish because


  They do not have a special mechanism to force water over the gills, but must keep swimming for ventilation to happen.

  Blood travels through the gills capillaries in the same direction as the water travels, described as parallel flow. Oxygen diffused from where it is more concentrated, in the water, to where it is less concentrated, in the blood. But this diffusion can only continue until the concentration are equal. So the blood's oxygen concentration is limited to 50% of its possible maximum value.

  Gas exchange in parallel flow does not occur across the whole gill lamella, only part of it, until the oxygen concentration in the blood and water is equal.
• Bony fish


  They have an internal skeleton made of bone and the gills are covered with a flap called the operculum, rather than opening directly on the side of the fish, as in cartilaginous fish.
• Ventilation


  To maintain a continuous, unidirectional flow, water is forced over the gill filaments by pressure differences. The water pressure in the mouth cavity is higher than in the opercular cavity. The operculum acts as both a valve, letting water out, and as a pump moving water past the gill filaments. The mouth also acts as a pump.
• Water in


  The mouth opens

  The operculum closes

  The floor of the month is lowered

  The volume inside the mouth cavity increases

  The pressure inside the mouth cavity decreases

  Water flows in, as the external pressure is higher than the pressure inside the mouth.
• Water out


  The mouth closes

  The operculum closes

  The floor of the the mouth lowered

  The volume inside the mouth cavity decreases

  The pressure inside the mouth cavity increases

  Water flows out over the gills because the pressure in the mouth cavity is higher than in the opercular cavity and outside.
• Bony fish have four pairs of gills


  Each gills is supported by a gill arch, sometimes called a gill bar, made of bone.

  Along each gill arch are many thin projections called gill filaments

  On the gill filaments are the gas exchange surface, the gill lamellae. These are held apart by weather flowing between them and provide a large surface area for gas exchange. Out of water they stick together and the gills collapse. Much less area is exposed and so not enough gas exchange can take place.
• Counter-current flow


  Water moves from the mouth cavity to the opercular cavity and into the gill pouches, where it flows between the gill lamellae. The blood in the gill capillaries flow in the opposite direction to the water flowing over the gill surface.

  The water always has a higher oxygen concentration than the blood, so oxygen diffuses into the blood along the whole length of the gill lamellae. This is a more efficient system than the parallel flow of the cartilaginous fish. The gills of a bony fish remove about 80% of the oxygen from the water.
• Structure of the humans breathing system

  The lungs are enclosed in an airtight compartment, the thorax

  Pleural membranes line the thorax and cover each lung. The fluid between the membranes prevents friction between the lungs and chest cavity as the lungs move.

  As the base of the thorax is a dome-shaped sheet of muscle, the diaphragm separating the thorax from the abdomen.

  The ribs surround the thorax

  The intercostal muscles are between the ribs.

  The trachea is a flexible airway, bringing air to the lungs.

  The two bronchi are the branches of the trachea.

  The lungs consist of a branching network of tubes called bronchioles, which arise from the bronchi.

  At the ends of the bronchioles the air sacs are called alveoli
• Ventilation of the lungs

  Mammals ventilate their lungs by negative pressure breathing. This means that for air to enter the lungs, the pressure inside the lungs must be below atmospheric pressure.
• Inspiration


  Breathing in
• Expiration


  Breathing out
• Steps in inspiration


  The external intercostal muscles contract.

  The ribs are pulled upwards and outwards.

  At the same time, the diaphragm muscles contract, so it flattens.

  Both actions increase the thorax volume.

  This reduces the pressure in the lungs.

  Atmospheric air pressure is now greater than the pressure in the lung, so air is forced into the lungs.
• Steps in expiration


  The external intercostal muscles relax

  The ribs move downwards and inwards

  At the same time, the diaphragm muscles relax, so it domes upwards

  Both actions decrease the thorax volume

  This increases the pressure in the lungs

  Air pressure in the lungs is now greater than the atmospheric pressure so air is forced out of the lungs.
• Why are alveoli efficient

  They provide a large surface area relative to the volume of the body.

  Gases dissolve in the surfactant moisture lining the alveoli.

  The alveoli have walls made of squamous epithelium, only one cell thick, so the diffusion pathway for gases is short.

  An extensive capillary network surrounds alveoli and maintains diffusion gradients, as carbon dioxide is rapidly brought to the alveoli and oxygen is rapidly carried away.

  The capillary walls also are only one cell thick, contributing to the short diffusion pathways for gases.
• Most adult insects are terrestrial and this means

  that water evaporates from their body surface and they risk dehydration.
• Efficient gas exchange requires a thin, permeable surface with a large area. Insects reduce water loss with


  a waterproof layer covering the body surface. In insects this is their exoskeleton which is rigid and comprises a thin waxy layer over a thicker layer of chitin and protein.